We show that steric sea-level varies with a period of 18.6 years along
the western European coast. We hypothesize that this variation
originates from the modulation of semidiurnal tides by the lunar nodal
cycle and associated changes in ocean mixing. Accounting for the steric
sea level changes in the upper 400 m of the ocean solves the discrepancy
between the nodal cycle in mean sea level observed by tide gauges and
the theoretical equilibrium nodal tide. Namely, by combining the
equilibrium tide with the nodal modulation of steric sea level, we close
the gap with the observations. This result supports earlier findings
that the observed phase and amplitude of the 18.6-year cycle do not
always correspond to the equilibrium nodal tide.